Conventionally, a substrate conveyance robot is used as a means for conveying substrates (plate members) such as semiconductor wafers and liquid crystal glass substrates. For example, the substrate conveyance robot has an articulated robot arm and an end effector (hand) provided to a distal end of the robot arm. The end effector has a substrate holding unit configured to hold a substrate thereon.
Usually, a plurality of wafers are stored in a substrate storing container such as a FOUP (wafer cassette). The substrate conveyance robot takes a wafer to be conveyed out from the inside of the substrate storing container and conveys the same to the side of a processing apparatus for treating the wafer. Alternatively, it takes a treated wafer out from a wafer holder on the processing apparatus side and stores the same inside the substrate storing container.
When taking a wafer out from the substrate storing container or the wafer holder using the substrate conveying robot, operation of the robot arm is controlled by a robot controller so as to insert the end effector into a vertical interval between wafers. From this state, the end effector is elevated so as to place a wafer to be conveyed on the end effector, and the wafer is fixed on the end effector by means of the substrate holding unit.
A wafer conveyance time from a starting point to a destination point by means of the substrate conveyance robot influences throughput of the wafer processing. Therefore, it is necessary to speed-up the robot operation so as to shorten the conveyance time in order to enhance throughput. Here, when trying to speed-up the robot operation so as to shorten the conveyance time, the acceleration (positive acceleration, negative acceleration) given to wafers at startups and stops of the operation is increased.
Accordingly, the substrate holding unit for fixing a wafer to the end effector is provided so that the wafer is not displaced from a predetermined position on the end effector or does not come off the end effector and fall, even when a large acceleration is given to the wafer.
Such substrate holding unit include a system in which the back surface of the wafer placed on the end effector is sucked by means of vacuum power (suction system), for example. Also, other substrate holding unit include a system in which an edge portion of the wafer placed on the end effector is engaged with a fixed engagement portion and a movable engagement portion so as to grip and fix the wafer by these engagement portions (edge grip system).
However, the wafer is sometimes insufficiently fixed to the end effector by the substrate holding unit for various reasons. When the wafer is insufficiently fixed, there is a risk that the wafer could be displaced from the predetermined position on the end effector during conveyance or comes off the end effector and falls.
In order to address this problem, a technology is proposed, which detects a displacement amount of the wafer from the predetermined position on the end effector, and when the displacement amount exceeds the predetermined threshold, restrains the acceleration of the robot arm operation and continues the conveyance operation (Patent Literature 1).